Hybrid vehicles use an electric motor for generation connected to an engine as the electric motor for engine startup. In this case, the electrical power source for driving this electric motor for generation involves a battery pack structured to connect a plurality of single batteries used when driving the electric motor of the vehicle. For example, some systems include a battery pack controller equipped with a detection circuit that detects whenever the voltage of any single battery drops below a given voltage, and when the detection circuit detects that the voltage of any single battery of the battery pack has dropped below the given voltage, controls the power supplied from the battery pack to the electric motor for starting the engine.